An organic electroluminescent element (hereafter, it is also called as an organic EL element) is a light emitting element having a constitution in which a light emitting layer containing a luminescent organic compound is interposed between a cathode and an anode. A hole injected from an anode and an electron injected from a cathode are recombined in the light emitting layer by applying an electric field, thus, an exciton is formed. It uses emitted light (fluorescence and phosphorescence) when the above exciton is deactivated. An organic EL element is a totally solid state element constituted by a film of an organic material having a thickness of only submicron and it enables to emit light at a voltage of several voltages to several ten voltages. Therefore, it is expected to be used for a flat display and an illumination of the next generation.
As a development of an organic EL device toward practical application, it was reported an organic EL element making use of phosphorescence emitted from an excited triplet state from Princeton University (refer to, for example, Non-patent document 1). Thereafter, there have been actively investigated materials emitting phosphorescence at room temperature (refer to, for example, Patent document 1 and Non-patent document 2). Further, organic EL elements operated by making use of phosphorescence emission make it possible to achieve a light emitting efficiency which is theoretically larger by about four times than those of conventional organic EL elements operated by making use of fluorescence emission. Therefore, starting from material development, a layer structure and electrodes of a light emitting element for the organic EL elements have been investigated and developed all over the world.
In the course of the research and development of the organic EL device, the most focused problem is the low durability of the organic EL device due to the fact the light emitting material itself is an organic compound. In order to improve the durability, many kinds of light emitting materials have been developed. At the same time, it has been made clear the importance of a host compound which receives and transfers an electron and a hole (they are generally called as a charge) to a light emitting material. The development of the host compound has been actively made (for example, refer to Patent documents 2, 3 and 4).
We have been focused on the clarification of the phenomenon in an interior of an organic EL element and we have eagerly analyzed the time-dependent change of the host compound contained in the light emitting layer. It was found that the fundamental primary factor which causes a various technical problems is a change of film resistance of the light emitting layer when applying current (to make luminescence) over time, and when keeping over time with non-luminescence condition.
Usually, it has been difficult to measure the resistance of a light emitting layer having a thickness of several tens of nm with a non-destructive method.
Recently, it has become possible to measure the resistance relatively easily by using impedance spectroscopy.
By using this method, it became possible to measure the resistance of a light emitting layer immediately after producing an organic EL element and at least one of the resistances during application of electric current and after keeping over time with non-light emitting state. It was found that the less the change of resistance, the less the voltage of the light emitting element, and that the chromaticity of the light emission spectrum is favorable. However, it was found that the change of resistance of a light emitting layer is still large and it is required to be further improved.